Capital gains or losses when new equity was issued due to merger

Hi.

I bought 600 shares of Corporation Bank from NSE in 3 tranches in 2010 and 2011. Total exp on purchase was 50135+681 (stt + brokerage) = 50816.

I received 198 shares of Union Bank of India in 2020 in lieu of 600 shares of Corporation Bank. 

I sold all 198 shares of UBI in 2025-26, for which the broker has given these details:

gross rate per security= 189.25 ;  total=37471.50

brokerage= 37.47  ;  GST= 6.96  ;  net amount= 37427.07

How should I compute the LTCG?

1. Should I show purchase of 198 shares of UBI in 2020 and include purchase expenditure (= 50816), then show sale in 2026 (=34427) and show capital loss (=50816-34427)?
2. OR should I see the share price of Corporation Bank on 31/03/2018, take that value as purchase price?
3. But the purchase quantity of UBI shares will not match (actually bought= 600 Corporation Bank, not of UBI)   

Kindly help!

[My problem arises because broker, ICICI Direct, says, this entry is to be manually given to them 'corporate action' to be included in LTCG statement. However, whenever I try to feed them the details through their utility, it gives an error. I can no longer wait for resolution at their end, and must give correct entries in my ITR myself.]